The aim of the study is to evaluate the effect of EGb761®, in comparison to placebo, on cerebral glucose metabolism, in three groups of elderly patients: newly diagnosed mild Alzheimer's disease (AD), memory complaint patients with cognitive impairment (MC) and memory complaint patients cognitively normal (CNE). The first phase includes four weeks treatment with EGb761® for all groups, with change in brain glucose metabolism at month 1 using 18 FDG-PET, as primary endpoint which will be followed by an open 17 months follow-up (FU) period with EGb761® treatment in MC and CNE patients.